Variability in plant flooding tolerance is often associated with differential growth
responses to increasing water depth. This study highlights how morphological responses
conferring flooding tolerance differ, using non-linear and quantile regression to
quantitatively compare flooding-related growth responses of three species.
Background and aims
Plant species composition in wetlands and on lakeshores often shows dramatic zonation,
which is frequently ascribed to differences in flooding tolerance. This study compared
the growth responses to water depth of three species (Phormium tenax, Carex
secta and Typha orientalis) differing in depth preferences
in wetlands, using non-linear and quantile regression analyses to establish how flooding
tolerance can explain field zonation.
Plants were established for 8 months in outdoor cultures in waterlogged soil without
standing water, and then randomly allocated to water depths from 0 to 0.5 m.
Morphological and growth responses to depth were followed for 54 days before harvest,
and then analysed by repeated-measures analysis of covariance, and non-linear and
quantile regression analysis (QRA), to compare flooding tolerances.
Growth responses to depth differed between the three species, and were non-linear.
Phormium tenax growth decreased rapidly in standing water
>0.25 m depth, C. secta growth increased initially with depth
but then decreased at depths >0.30 m, accompanied by increased shoot height and
decreased shoot density, and T. orientalis was unaffected by the 0- to
0.50-m depth range. In P. tenax the decrease in growth was associated
with a decrease in the number of leaves produced per ramet and in C.
secta the effect of water depth was greatest for the tallest shoots.
Allocation patterns were unaffected by depth.
The responses are consistent with the principle that zonation in the field is primarily
structured by competition in shallow water and by physiological flooding tolerance in
deep water. Regression analyses, especially QRA, proved to be powerful tools in
distinguishing genuine phenotypic responses to water depth from non-phenotypic variation
due to size and developmental differences.